Green Team Part 18: Why Grow This, When You Can Grow That?

Zeina G Zahalan

As landscape architects we view soil as the foundation for plant life. But there are many other factors to consider when designing with plants. As any novice landscape designer will tell you, it’s easy to feel overwhelmed by the abundance of plants and plant varieties. This is why we need to understand every existing site, its conditions, and its context.

Several factors go into the selection of plant species, such as sensitivities to sun, shade, wind, soil pH, soil chemistry, and the availability of water. This is why our initial evaluation of a site includes inventory and analysis, critical to making informed decisions about plant selection and design.

Inventory

For a recent planting design project at a west side Manhattan co-op, my first step was to take an inventory. I observed and recorded all existing plant material, noting its condition. In the process, I noticed that throughout the site, the planting scheme consisted of Euonymus kiautschovicus “Manhattan” and a variety of Taxus x media, which rendered a monochromatic color scheme that lacked in seasonal interest. Here, then, was an opportunity for improvement!

The existing plant material comprised a variety of evergreen plants with no significant flower, demonstrating the lack of seasonal interest.

Courtesy Mathews Nielsen

I observed, furthermore, that in many areas the soil was compacted, causing distress to the plants. Clearly, this issue needed to be addressed. Using an existing survey of the site, we verified and confirmed infrastructure locations. Such fixed constraints are often unseen but must be considered when developing the plant palette and layout of plant material.

Analysis

Following the initial inventory of the landscape, we analyzed the site in terms of circulation, window heights, environmental factors (sun, wind, etc.), and soil chemistry. It is best to design walkways and routes so that users can understand the links on a site. In the case of the urban co-op, we decided to locate all entrances according to their frequency of use, prioritizing the areas that could be most substantially improved to best benefit the residents. This analysis would also determine the areas that would require lower height plant material, ensuring clear views that maintain sight lines and visible access for residents and visitors alike, as well as to prevent users from becoming disoriented.

The primary, secondary, professional and maintenance entrances as well as infrastructure and pedestrian circulation.

Courtesy Mathews Nielsen

A most important assessment involves evaluating the pattern of the sun’s trajectory. In New York City with its large areas of dense shade due to the massing of buildings and skyscrapers, we need to make sure that the plants we use get some sunlight to support photosynthesis. This particular analysis focuses on the summer solstice to determine the types of plants that require full sun, part sun, part shade, or full shade. To my surprise, our solar analysis revealed more areas of sunlight than we anticipated, some at the entrances, others on the backside of entrances, and still others in gathering areas. This finding expanded my planting palette beyond shade tolerant species.

This image illustrates the pattern of the sun at the summer solstice. The light blue areas receive +6 hours of sun, teal color receives 2-4 hours of sun, and the dark blue receives less than 2 hours of sun.

Courtesy Mathews Nielsen

After reviewing existing soil conditions, we felt it necessary to have the soil tested and analyzed, in order to understand its chemical and structural character. Luckily, we found that its pH levels and nutrient content were within acceptable ranges. So we only needed to recommend using compost, which was incorporated by roto-tilling in order to de-compact and re-establish a healthy soil structure.

The inventory and analysis act as part of the criteria that establish a basis for design. Once these are complete, the planting design process can begin. In addition, client concerns are equally important. For example, a problem we encountered involved rodents from adjacent subway stations seeking shelter and food on our site. We had to address this, plus the client’s and residents’ desire for more seasonal plantings.

Providing a splash of color through curvilinear plant forms that juxtapose the orthogonal building shapes invigorates the entrance.

Courtesy Mathews Nielsen

This image illustrates the seasonal characteristics and size of the proposed plant material in the spring season.

Courtesy Mathews Nielsen

The client also requested plants that would attract beneficial wildlife. Our selection of plant materials accomplished these goals, and some things bloom throughout the seasons. Examples include those plants that attract birds and butterflies, and have great fall foliage:

Some of the plant material mentioned above, planted in an area that acts as the main entrance for residents into the building, allowing for visibility in and out of it.

Courtesy Mathews Nielsen

While these plants offer habitat benefits, they also discourage rodents. After looking into ways to deal with this common problem in big cities, I came across Nepeta x faassenii, known as Catmint, which acts as a natural repellent to rodents as well as a variety of insects, including mosquitos. With its aromatic grayish foliage and continued tall violet tubular blooms, Nepeta x faassenii nestles well between the fine texture of Sesleria autumnalis (Autumn Moor Grass).

A swath of Nepeta x faassenii is planted through the middle of an area that consistently attracted rodents.

Courtesy Mathews Nielsen

Clearly, a variety of criteria need to be analyzed to ensure that a planting design is aesthetically appealing as well as long lasting. After a design is implemented, it takes one to three years for the plant life to fully establish itself, and even more to grow to its mature size, which we discussed in Planting for the Future.

While aesthetics are a key aspect of any design, it is imperative that a material—plant or otherwise—be selected for use on that unique site and be suited to its conditions. It is clear to us that plant selection needs to be geographically appropriate, not based solely on the “look” of the species.

In our next post, Terrie Brightman asks, “What’s so great about …bamboo?”

Zeina G. Zahalan, ASLA, joined Mathews Nielsen Landscape Architects in January 2012. She holds a bachelor of science in landscape architecture from Rutgers University. While pursuing her degree, Zeina worked extensively on streetscape and community garden designs for community development corporations in New Jersey. She is the recipient of the Roy deBoer Travel Grant, the American Society of Landscape Architects Council of Fellows Award, and the New Jersey American Society of Landscape Architects Student Honor Award.

This is one in a series of Metropolis blogs written by members of Mathews Nielsen Landscape Architects’ Green Team, which focuses on research as the groundswell of effective landscape design and implementation. Addressing the design challenges the Green Team encounters and how it resolves them, the series shares the team’s research in response to project constraints and questions that emerge, revealing their solutions. Along the way, the team also shares its knowledge about plants, geography, stormwater, sustainability, materials, and more.

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